1. Field of the Invention
The present invention relates to a method of and an apparatus for transmitting image information and, more particularly, to improved image information transmission method and apparatus which provide a high level of coding efficiency.
2. Description of the Prior Art:
Hitherto, a high-efficiency television signal coding method has been known as a method for transmitting image information. In order to restrict the transmission band, this method makes use of a so-called MAX-MIN method which relies upon minimization of the average number of picture elements as will be understood from the following description.
In general, television signal has a very close time/space correlation. When an image is divided into minute blocks, the dynamic range of each block is often restricted by a local correlation. It is therefore possible to compress information in a very efficient manner by determining the dynamic range for each block and coding the information of each block in an adaptive manner.
The coding operation will be described with reference to FIGS. 1 to 6.
FIG. 1 is a schematic block diagram of a known system for transmitting image information. The image information transmission system has an input terminal 101 which receives digital image data obtained through sampling, at a predetermined frequency, a raster-scan analog image signal such as a television signal and digitizing the sample data such that each sample comprises n bits. Thus, the input digital image data has a gradation represented by 2.sup.n. The input data is delivered first to a picture element block division circuit 102 which is capable of dividing the data carried by all picture elements on the whole picture frame into a plurality of blocks.
The division of the picture element data on the whole picture frame into blocks is conducted in a manner which will be explained with reference to FIG. 2. The picture element block division circuit 102 has a memory or an equivalent means which temporarily stores the image data carried by all the picture elements on a picture frame. The circuit 102 then divides the stored data into a plurality of blocks each having l picture elements in the horizontal direction (referred to as the "H" direction) and m picture elements in the vertical direction (referred to as the "V" direction), i.e., (l.times.m) blocks in total, and reads the picture element data on block basis, i.e., in a block-by-block fashion.
FIG. 3 illustrates the construction of each picture element block. It will be seen that each block contains picture elements 1.1 to m.l which carry data D.sub.l.1 to D.sub.m.l.
The image data output from the picture element block division circuit 102 is delivered to a maximum value detecting circuit 103, a minimum value detecting circuit 104 and a timing control section 105. In consequence, the data Dmax and Dmin having the maximum and minimum values are detected by the maximum value detecting circuit 103 and the minimum value detecting circuit 104, respectively. The timing control circuit 105 has the function of delaying the whole picture element data by a period which is long enough to enable the data Dmax and Dmin having the maximum and minimum values to be detected, and delivers the picture element data of each picture element block to a division value conversion circuit 106 in a predetermined sequence. For instance, picture element data are transmitted from the successive picture elements of each group in a sequence which is expressed as D.sub.l.1, D.sub.2.1, D.sub.3.1, ..., D.sub.m.1, D.sub.l.2, ...., D.sub.m.2, ....., D.sub.l.(l-1), ...., D.sub.m.( l-1), D.sub.l.l ...., D.sub.m.l. Thus, the division value conversion circuit 106 receives the picture element data D.sub.l.1 to D.sub.m.l) carried by all the picture elements in each picture element block, as well as the maximum and minimum values D.sub.max and D.sub.min from among these data. The division value conversion circuit 106 compares each picture element data with quantized levels which are obtained by dividing the difference between the maximum and minimum data Dmax and Dmin by 2.sup.k (k represents an integer smaller than n), whereby k-bit division codes (.DELTA..sub.1.1 -.DELTA..sub.m.l) are obtained. The quantization is conducted in a manner which will be described with reference to FIG. 4(a).
As will be seen from FIG. 4(a), the division codes .DELTA..sub.i.j are output in the form of k-bit binary codes. The k-bit division codes .DELTA..sub.i.j and n-bit data Dmax and Dmin are respectively converted into serial data by parallel-serial (P-S) conversion circuit 107, 107' and 107". These serial data are then formed into serial data as shown in FIG. 5 by means of the data selector 108. It will be understood that the serial data shown in FIG. 5 corresponds to one picture element block.
The data output from the data selector 108 is input to a first-in first-out memory (FiFo memory) 109 which conducts time-axis processing on the input data so as to provide a constant data transmission rate. The output from the FiFo memory 109 is input to a sync signal adding circuit 110 so that sync signals are added to the data. The data thus provided with sync signals is then delivered through an output terminal 111 to a transmission line which leads to, for example, a magnetic recording/reproduction system of a VTR. The addition of the synchronizing signal may be conducted on each picture element block or on each group of a predetermined number of picture element blocks. The timing of operation of each element of the circuit is determined in accordance with a timing signal which is output from a timing control circuit 112.
FIG. 6 is a block diagram which schematically shows the construction of a circuit which receives the data signal from the signal transmission end having the construction shown in FIG. 1. The circuit includes an input terminal 621 which receives the data which has been coded at high efficiency as explained before in connection with FIG. 1. The synchronizing signal included in the transmission data is separated by a sync signal separation circuit 622 and is delivered to a timing control circuit 623. The timing control section determines, in accordance with the separated sync signal, the timing of operation of each element of the circuit of FIG. 6.
The circuit also includes a data selector 624 which divides the transmitted data into n-bit data Dmax, Dmin and the codes .DELTA..sub.i,j which have been obtained by quantizing the respective picture element data into k-bit codes between the maximum and minimum data Dmax and Dmin. These data are converted into parallel data through S-P (serial to parallel) conversion circuit 625 and 625'. The maximum and minimum data Dmax and Dmin among the picture element data in each picture element block, converted into parallel form through the S-P conversion circuit 625, are latched by latch circuits 626 and 627, respectively, and the thus latched data are transmitted to a division value inverse conversion circuit 628. On the other hand, the division codes .DELTA..sub.i,j relating to the respective picture element data in each picture element block are output from the S-P conversion circuit 625' in a predetermined sequence as explained before and are supplied to the division value inverse conversion circuit 628.
FIG. 4(b) shows the manner in which the representative value data D'.sub.i,j relating to the original picture element data are decoded from the division codes .sub.i,j and the maximum and minimum data Dmax and Dmin. As illustrated, the representative values are set at levels intermediate between adjacent quantized levels obtained by dividing the difference between the maximum and minimum data Dmax and Dmin by 2.sup.k. The n-bit representative data (D'.sub.l,l -D'.sub.m,l) thus derived from the division value inverse conversion circuit 628 are output in the aforementioned sequence in block-by-block fashion. The output data from the division value inverse conversion circuit 628 is input to a scan conversion circuit 629 which changes the sequence of the output data to a sequence corresponding to raster scan, whereby decoded image data is obtained and output through the output terminal 630.
The described apparatus makes use of only two-dimensional space correlation of the image. Therefore, certain levels of redundancy on the time axis are inevitably caused during the transmission of image information, particularly when a still image or an image which is substantially motionless is transmitted. Namely, data which are substantially the same are repeatedly transmitted with the result that the transmission efficiency is undesirably decreased.